Semiconductor processing involves a number of different chemical and physical processes whereby minute integrated circuits are created on a substrate. The integrated circuits are constructed using multilayers of interrelated patterns of various materials, the layers being created by such processes as chemical vapor deposition (CVD), physical vapor deposition (PVD), and epitaxial growth. Some layers are patterned using photoresist masks and wet and dry etching techniques. Patterns are created within layers by the implantation of dopants at particular locations. The substrate upon which the integrated circuit is created may be silicon, gallium arsenide, glass, or other appropriate material.
In the production of semiconductor workpieces, plasma etching and reactive ion etching (RIE), both of which employ reactive gas plasma, are presently the most widely used processes to form fine wire patterns in the submicron line width region. In general, plasmas provide higher etch rates, greater anistropy (i.e. more vertical profiles), and lower foreign material concentrations as compared to wet etchants. In plasma etching, a gas (or combination of gases) is ionized to form a plasma. Depending on the conditions of the system (e.g., pressure, power, bias to the electrodes, etc.) as well as the nature of the ions, the exposed material can be etched in a "physical" mode, a "chemical" mode, or a mode that is both physical and chemical. In the physical etch mode, the ions are inert with respect to the exposed materials, but have sufficient energy to physically dislodge atoms from the exposed surface. In the chemical mode, the ions do chemically react with the exposed surface to form gaseous reaction products that are prepared from the chamber. In RIE both physical and chemical etching take place.
In present plasma etching processes requiring submicron patterns, the semiconductor workpiece substrate to be processed is placed on an electrode pedestal and is surrounded by a focus ring which is a cylindrical insulator. The focus ring functions to enhance the uniform application of the etch reaction of the plasma on the surface of the semiconductor workpiece/substrate being processed. Generally the progress of the etching reaction is slower at the center portion of the workpiece than at the peripheral portion thereof. This is due to an "internal loading effect" which refers to the depletion of the etching reaction seeds in the center portion of the workpiece/substrate due to the etching reaction. The focus ring functions to decrease the progress and speed of the etch reaction at the peripheral portion of the substrate, thereby achieving a substantial etching uniformity over the entire workpiece/substrate surface.
Many of the processes carried out within semiconductor processing systems leave contaminant deposits on the elements of the process reactor and the reactor chamber walls which accumulate and become the source of particulate matter harmful to the creation of a semiconductor device. As the geometries of semiconductor devices become ever so smaller, the ability to maintain the uniformity and accuracy of critical dimensions becomes strained. In this dimensional downsizing environment, the avoidance of contaminant particulate matter upon the surface of the semiconductor workpiece has become more critical.
Particulate contamination buildup on semiconductor process chambers and other reactor elements such as focus rings is particularly significant in the etch processing of semiconductor elements employing metal films. These metal films are generally etched by employing a number of reactive gases, including halocarbon gases, as plasma components. In the case of an aluminum film, the etchant gases used are predominantly the chlorine containing gases, chlorine (Cl.sub.2) and boron trichloride (BCl.sub.3), which enables formation of volatile aluminum chloride compounds upon etching, which volatile compounds can be removed from the etch processing chamber by applied vacuum. However, simultaneously with the formation of volatile aluminum chloride compounds, other active chlorine and boron containing species are formed which can react with any oxygen and water vapor present in the etch processing chamber or with organic species from pattenting photoresist to form non volatile particulate compositions which ultimately produce relatively large quantities of contaminant on the inner walls of the process chamber. The non volatile particulate compositions initially tend to remain inside the etch chamber in the form of loosely attached particles to the chamber etch surfaces. These loosely attached etch by-product compounds can easily break free of the surface to which they are attached and fall upon a workpiece/substrate surface causing contamination of the workpiece surface, thereby resulting in a defective device.
This problem of contaminant generation and buildup becomes more acute in metal etch processes employing a focus ring because the proximity of the cylindrical walls shrouding the workpiece favor and enhance deposition and coating of this wall surface with contaminants. Generally, the non-volatile compositions generated in metal etch processes combine with polymeric materials formed from photoresist and carbon containing etchant gases (as by-products of the etch process) and accumulate to form a contaminant coating on the inner wall of the cylindrical focus ring. As the thickness of this contaminant coating increases, stability of the deposited layer decreases due to its weight and/or the stress force exerted on the coating from the curvature of the cylindrical wall, eventually resulting in cracking and excessive particulate flaking or powdering form the coating surface. The powder or flakes drop off the cylindrical walls of the ring thereby causing contamination of the semiconductor workpiece/substrate. As in the case of any semiconductor process system, the apparatus employed in metal etch must be cleaned periodically in order to avoid these problems and, of course, such cleaning requires shutdown of the plasma operation with consequent loss of production.
Known plasma chamber cleaning methods have involved opening the plasma etch chamber, disassembling portions of the chamber, and removing the contaminant deposits by physical of chemical methods. For example, the chamber can be rinsed with a solution of hydrochloric acid, or hand wiped with a solvent, to dissolve various contaminants. The etch chamber alternatively may be washed with water and dried. The same cleaning techniques are separately applied to the vacuum conductance channels and pump system to avoid the inevitable diminished vacuum or suffocation referred to above. All of these cleaning methods are complicated, disruptive, time consuming and can be sources of additional contamination.
Plasma enhanced dry cleaning processes exist whereby contaminants attached to the inside walls of a focus ring or a film deposition reaction chamber are removed by plasma etching using carbon tetrachloride and oxygen. However, presently known plasma enhanced dry cleaning systems require a dry cleaning time period equal to about 5% to 10% of the time spent in the aluminum etching process itself. Moreover, the dry cleaning plasma processes are generally ineffective with respect to the vacuum exhaust system which would have to be separately cleaned. It would clearly be advantageous to delay cleaning of plasma etch process chambers and the present invention effects such a result by providing a focus ring which controls and stabilizes the buildup of contaminant coatings thereon.
Because a focus ring has proximity to the workpiece/surface substrate and, consequently, is more susceptible to contaminant build-up in plasma etch processing, it is desirable to provide a focus ring which accommodates and stabilizes coatings of contaminant residues and requires less frequency of cleaning.